The present invention relates generally to liquid developers and methods of making liquid developers for use in electrostatographic reproducing systems. In particular, the present invention is directed to a liquid developer and a method for making the liquid developer which employs a novel marking particle, and specifically one which is of improved colored density with excellent shelf life stability and good fixing characteristics.
In the electrostatographic reproducing process in most common commercial use today, xerography, a light image of an original to be copied is typically recorded in the form of an electrostatic latent image upon a photosensitive member. The electrostatic latent image may be rendered visible by the application of electroscopic marking particles, referred to in the art as toner. The toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support such as a sheet of plain paper with subsequent affixing of the image thereto.
An alternative development technique to that described above involves the use of a liquid developer or liquid toner. The conventional commercial liquid toners in present use in automatic office reproducing machines generally constitute a dispersion of pigments in a liquid hydrocarbon. Once the electrostatic latent image is formed, which is typically on a single use sheet of photoconductive paper, such as zinc oxide, it is transported through a bath of the liquid developer. When in contact with the liquid developers, the charged pigment particles in the liquid developer migrate through the liquid to the sheet in the configuration of charged image on the imaging sheet. The sheet may then be withdrawn from the liquid developer bath with the charged particles adhering to the electrostatic latent image in image configuration and a thin film of the residual developer remaining on the surface of the paper being evaporated within a few seconds. If desired, the marking particles may be fixed to the sheet in an image configuration.
Liquid toners of the present invention however are not to be understood to be limited to field of application in the xerographic process. They may, for example, be used in a variety of reproduction process including among others, electrographic recording, electrostatic printing, and facsimile printing. Accordingly, it should be appreciated that the description which herein follows is applicable to liquid developers in general, which may have utility in a variety of commercial embodiments.
As mentioned above, the liquid developers presented a first alternative to dry toner development of electrostatic latent images in automatic reproducing machines. In their earliest application they took the form of a pigment, such as carbon black, which would be dispersed in a petroleum distillate and have a charge applied thereto with a charge control agent such as a metal soap. The problem with the earliest liquid developers existed in their dispersion stability in that upon being stored for any extensive period of time, the carbon black pigment would tend to settle out of the dispersion medium and flocculate into nonredispersable macroscopic material at the bottom of the vessel. In an attempt to overcome this difficulty, a dispersant such as polyisobutylene which was soluble in the carrier liquid and which would be adsorbed on the carbon black pigment particles, was added in an attempt to provide a steric barrier between the individual particles. In effect, this was an attempt to provide increased dispersion stability by increasing the repulsive interaction between the individual carbon black particles, and to provide a more uniform dispersion so that the particles would not settle out. it was believed that the presence of the resin maintained the carbon black as discrete particles over long periods of time by providing a protective coating for the carbon black particles so that the attractive forces between adjacent particles would not come into play. While this was a dramatic improvement over the liquid developers without a dispersant that had been used heretofore, they suffered the difficulty in that the resin coating in some instances intended to desorb from the carbon black particles thereby permitting the attractive forces between adjacent particles to once again come into play. This resulted in the individual carbon black particles flocculating and settling to the bottom of the dispersion vessel.
The next step in the evolution of the development of liquid developers involved the use of amphipathic copolymers. For example, instead of the polyisobutylene homopolymer dispersant above which was soluble in most of the aliphatic hydrocarbons that were used as dispersion vehicles and which also coated the carbon black, an amphipathic copolymer which could be a block or graft copolymer was prepared on the theory that part of the copolymer would have an affinity for the liquid phase, the hydrocarbon liquid, and part of the copolymer would have an affinity for the surface of the individual pigment particles. Thus with the use of such an amphipathic copolymer, the part of the copolymer that wants to separate is adsorbed on the carbon black particle surface and binds the soluble part of the polymer to the particle surface thereby reducing the desorption of the polymer from the carbon black particles. Typical such approaches are those described in U.S. Pat. Nos. 3,554,946 (Okuno et al.), 3,623,986 (Machida et al.) and 3,890,240 (Hockberg). Even with this improvement in liquid developers, the dispersion stability continues to remain a problem, in that it was always possible that the stabilizer will be desorbed from the particle surface rendering the developer thermodynamically unstable.
The next event in the development of liquid developers involved trying to make a developer wherein desorption of the dispersant was in effect theoretically impossible. It was believed that a stable liquid developer would be provided if the particle contained a steric barrier which could not be desorbed from the particle surface. This of course is very difficult to do in the chemical sense when one is dealing with a carbon black pigment. The way around this particular difficulty however is to chemically make a particle wherein the steric barrier is chemically tied to the particle surface. This is typically done with a non-aqueous dispersion of polymer particles wherein a steric barrier is attached to the polymer surface thereby providing a thermodynamically stable polymer particle. This provides a liquid developer wherein the individual marking particles do not flocculate.